The present invention relates in general to turbo-machinery. More particularly, the present invention relates to reducing fatigue failure of fan blades for axial flow turbo-machines such as air cycle machines.
Environmental control systems including air cycle machines have long been used in military and commercial aircraft to supply air at flow rates desirable for heating, cooling and pressurizing aircraft cabins. Air cycle environmental control systems are light in weight and have low maintenance requirements. Additionally, air cycle environmental control systems use clean working fluid--compressed air--instead of refrigerants that are harmful to the environment.
Greater comfort demands of customers and higher heat loads (due to a greater on-board usage of personal electronics and other heat-generating devices) have significantly increased the demand for higher cooling capacities of the air cycle machines. To satisfy this demand, efforts have been directed towards increasing fan flow rates of axial flow air cycle machines. Increasing the length to width ratio (i.e. increasing the aspect ratio) of the fan blades and increasing the spacing between the fan blades (i.e., lowering the solidity) have been proposed to increase flow rates without increasing fan diameter (for those air cycle machines having fans) and without decreasing air cycle machine performance.
However, problems with fatigue failure have been reported for high aspect ratio, low solidity blades. Constant cycle reversals of bending and torsion coupled with transonic/supersonic flutter have been determined to cause cracks and subsequent breaks in the blades. A broken blade can create an imbalance in the air cycle machine. The imbalance, in turn, can damage the journal and thrust bearings and cause them to seize. If the journal and/or thrust bearings seize, the air cycle machine is either rebuilt or replaced.
Fatigue life of high aspect ratio, low solidity fan blades could be increased by increasing blade thickness and using larger fillet radii. However, this approach adds to the size and weight of the air cycle machine and, therefore, is undesirable. Moreover, this approach involves redesign and fabrication of new fan blades, but does not address potential fatigue failure problems facing high aspect ratio, low solidity fan blades of air cycle machines that have already been built and sold to customers.